The Russian military on Sunday said that the United States had struck Syria's Idlib through the air, accusing the American army of putting the cease-fire reached over the weekend at risk.
The Russian army also reported that U.S. airstrikes had resulted in casualties.
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On Saturday, a monitor and rebel official said that Idlib saw a lull in air raids that have been pounding the insurgent territory after the government said it was ceasing fire.
It is the second such truce declared this month in Idlib, which is in Syria's northwest corner - the only major swathe of the country still in rebel hands after over eight years of war. A ceasefire in early August collapsed three days in, after which the Russian-backed army pressed its offensive and gained ground.
The Syrian government agreed to a ceasefire that started on Saturday morning "while reserving the right to respond to any violation by the terrorists," the state news agency SANA said.